1. Field of the Invention
The present invention concerns techniques for elastography and a magnetic resonance system for implementing those techniques. More particularly, the invention concerns techniques of elastography in which shear waves are generated by a radio-frequency coil array of such a magnetic resonance system.
2. Description of the Prior Art
Elastography is a medical examination modality that allows a determination of tissue elasticity to be made. For example, magnetic resonance elastography (MRE) is a known technique that allows properties of shear waves (transverse sound waves in a frequency range of approximately 10-200 Hz) within body tissue to be detected and imaged by means of magnetic resonance signals. The shear waves are applied or generated on the outer surface of the body, for example by a mechanical vibration device. Their propagation is monitored within the body tissue by means of magnetic resonance signals that are caused to be emitted from the tissue while the waves are propagating therein. Properties of the shear waves can be indicative of a tissue rigidity or tissue elasticity. Therefore, clinical applications and diagnostics can be implemented based on the measurement of these properties.
Such MRE techniques are known from multiple sources, for example from the US Patent Application Publication No. 2003/0193336 A1. Harmonic shear waves are externally applied in order to bring the tissue into oscillation. The displacement of the tissue that results can then be detected by means of suitable magnetic resonance (MR) measurement sequences. The tissue elasticity can be quantified from the detected MR signals with a spatial resolution. Typical frequencies of the shear waves that are used are 20 Hz-200 Hz, for example. In MRE, movement-coding gradient fields (“motion-sensitizing gradients”) can be used to image the tissue displacement due to the shear waves, for example.
A technically related method known as “acoustic radiation force imaging” (ARFI) images representing tissue displacement that are generated by focused ultrasound pulses with high intensity (“high intensity focused ultrasound, HIFU”). With such ARFI techniques it is possible to use focused ultrasound, i.e. longitudinal shear waves in a frequency range of greater than 1 MHz (pressure wave), to generate shear waves with relatively low frequencies. Motion-sensitizing gradient fields can also be used for imaging the tissue displacement.
An additional technique to image focused ultrasound pulses is known from US 2010/0026298 A1 and U.S. Pat. No. 7,956,613 B2.
Spin-lock measurement sequences are used in connection with focused ultrasound pulses to image the tissue displacement, such as for the purpose of destroying tumors by means of thermal ablation.
Such known techniques have a number of disadvantages. For example, the shear waves or ultrasound waves are typically generated and applied at the surface of the body by means of mechanical vibration devices or by means of ultrasound emitters. In general, these devices for generation are not compatible (or are compatible only to a limited extent) with the environment within an MR system. Susceptibility artifacts can arise in the MR signals that are generated in the presence of such devices, or specially designed and thus expensive equipment must be used that is more MR-compatible. Special solutions are then required that complicate the clinical workflow and produce further limitations with regard to the space available to the patient within the magnetic resonance tube. Moreover, the additional components that are necessary produce increased costs in implementation, as well as increased costs for procurement and servicing.
Space problems also can arise due to the use of mechanical vibration devices or ultrasound emitters. Within the scope of a magnetic resonance measurement (data acquisition) sequence, it can be desirable to place a local coil or a local coil array near the surface of the body of the patient. Since the mechanical vibration devices or ultrasound emitters are most advantageously placed at such locations, the space problems can result.
Moreover, shear waves and/or ultrasound waves exhibit a relatively high attenuation during propagation through the tissue. For example, from experiments it is known that shear waves above an eigenfrequency of 200 Hz (which corresponds to a wavelength of 1 to 3 cm) are absorbed rapidly within the tissue and therefore cannot be used in deep tissue layers (i.e. far removed from the site of generation at the skin surface of the patient). This is because shear waves or ultrasound waves are generated by mechanical vibration devices or ultrasound emitters located on the surface of the patient, and must first travel a certain distance within the tissue before they arrive at the relevant sample site or measurement region.
The use of low-frequency shear waves (for example below 35 Hz, which corresponds to a wavelength of 6 to 17 cm), however, significantly reduces the achievable spatial resolution. For example, this can hinder or make impossible the detection of smaller tumors, or generally limit the diagnosis evaluation capability. Moreover, given the use of such low-frequency shear waves, the contrast in MRE images is not solely dependent on the acoustic properties of the tissue, but also is a function of the resulting wavelength within the body. For example, standing waves can arise in resonators that are formed by the tissue structure. This can complicate the clinical interpretation of the MRE images or make it impossible.